1. Technical Field
The present invention relates to a fibrous separation membrane for secondary batteries, and a method of manufacturing the same.
2. Description of the Related Art
A secondary battery is a battery which can be reused because it can be recharged using external energy and returned to an original state after being discharged.
Such a secondary battery is characterized in that it has high power density, it can do high-power discharge, and it is only slightly influenced by temperature.
Recently, green energy has attracted considerable attention, and thus secondary batteries have expanded their fields to IT, EV, ESS, and the like.
The demand for secondary batteries is rapidly increasing, and the function of secondary batteries is also becoming highly functionalized.
Such a secondary battery includes the four major components of a cathode active material, an anode active material, an electrolyte and a separation membrane. Among them, a separation membrane serves to separate a cathode active material and an anode active material, and is used as an ion transfer passage. As such, since a separation membrane serves to provide an ion transfer passage and prevent foreign matter from moving, it must have pores having a size of several micrometers or less.
Conventional separation membranes are mostly formed by a wet process or a dry process.
The wet process is a process of forming pores by phase-separating a solution containing a polymer, a solvent and other components and then stretching the phase-separated product, and the dry process is a process of forming pores by extruding a polymer and then stretching the extruded polymer.
Since biaxial stretching must be conducted in the wet process, the wet process is advantageous in that pores are non-oriented, but is disadvantageous in that manufacturing costs are high. In contrast, since uniaxial stretching must be conducted in the dry process, the dry process is disadvantageous in that pores are oriented, but is advantageous in that manufacturing costs are low.
All of the separation membranes formed by the wet process or the dry process are made of polyolefin-based resins. The separation membrane is generally made of two kinds of resins, namely polyethylene and polypropylene. The separation membrane is produced by mixing the two kinds of resins or laminating them.
The separation membrane made of polyethylene is advantageous in the shutdown of a secondary battery because of its low melting point, but is problematic in that it is greatly thermally constricted at high temperature because its melting point is low. In contrast, the separation membrane made of polypropylene is advantageous in thermal constriction, but is disadvantageous in the shutdown of a secondary battery because of its high melting point.